1. Field of the Invention
The invention relates to light-scattering polystyrene molding compounds having embedded crosslinked particles comprised of methacrylate copolymers; and molded articles produced from said molding compounds.
2. Discussion of the Background
Inorganic pigments such as titanium dioxide, barium sulfate, or silicic acid are commonly used to cloud polymer materials used to produce diffusely scattering molded articles. When these pigments are employed in sufficient concentrations and with suitable mean particle size and index of refraction, satisfactory diffusion of transmitted light can be achieved; but the light transmissivity is sharply reduced. A further disadvantage of inorganic pigments is that sedimentation occurs during polymerization which results in a nonuniform distribution of pigment in the molded article rendering the physical properties of the polymer material unsatisfactory.
In view of the preceding, dimensionally stable polymer particles have replaced inorganic pigments, however there is a substantial difference between the index of refraction of said particles and that of the polymer matrix.
U.S. Pat. No. 4,464,513 describes a clouded polyvinyl chloride (PVC) molding compound which contains crosslinked polymer particles with particle sizes between 35 and 500 micron, in amounts of up to approximately 30 weight %. The polymer particles are comprised of vinylaromatic, alkyl acrylate, alkyl methacrylate, or possibly other units of monoethylenically unsaturated monomers, and units of a crosslinking monomer which has at least two double bonds. The scattering particles are also disclosed as being useful for other thermoplastic molding compounds.
U.S. Pat. No. 4,594,363 (European Patent 188,325) describes scattering particles with a two-phase core-and-shell structure, wherein the core and shell comprise monomers having acid groups. These dispersed scattering particles are swelled with volatile bases which evaporate upon drying and leave hollow spaces in the scattering particles. The scattering particles are preferably used as replacements for inorganic pigments in dispersion dyes and in thermally hardening lacquer systems.
European Patent 342,283 describes spherically-shaped, 2-15 micron scattering particles with multiphase structure, wherein the scattering particles may be present in proportions of up to 40 weight % in a thermoplastic polymeric matrix phase. The indices of refraction of the scattering particles and the matrix phase differ by at least 0.003, and at most 0.2. Preferably, the scattering particles are composed of units of acrylates and methacrylates, and the matrix phase is composed of polymers with a higher index of refraction, such as PVC or polystyrene.
Japanese Patent Application 61-159,440 describes 30-300 micron scattering particles dispersed in polymethyl methacrylate (PMMA) in proportions up to 30 weight %. The scattering particles are composed of a copolymer of vinylaromatic, alkyl acrylate, and alkyl methacrylate units, and units of crosslinking comonomers.
German Patent 35 28 165 (U.S. Pat. No. 4,876,311) describes clouded plastic elements containing 20-50 micron beads of crosslinked polymers in proportions of 3-30 weight % (based on the weight of the polymer matrix). The polymer beads serve as scattering particles and are composed of aromatic-containing or halogen-containing monomers, vinyl monomers copolymerizable with said aromatic-containing or halogen-containing monomers, crosslinking monomers, and (optionally) strongly polar monomers. The polymer matrix, typically acrylic resins, must have an index of refraction differing from that of the scattering particles by at least 0.02.
Scattering particles for methacrylic resins comprised of copolymers of methyl methacrylate, butyl (meth)acrylate, and a crosslinking agent (e.g. allyl methacrylate), are described in Japanese Patent Application 82-14,650.
German Patent 2,146,607 (U.S. Pat. Nos. 3,992,486 and 4,000,216) claims polymer mixtures which are translucent to opaque and are composed of a continuous phase composed of a soluble uncrosslinked polymer produced by polymerization of (meth)acrylic acid esters, styrenes, vinyl chloride, or vinyl acetate, in which there are dispersed crosslinked polymer particles composed of 87-99.99 weight % of at least one mono-ethylenically unsaturated monomer from the group of the (meth)acrylic acid esters or the styrenes, and 0.01-3 weight % of at least one multiply unsaturated monomer, and (optionally) 0-10 weight % of a polar monomer. The scattering particles are practically spherical in shape and have particle sizes of 0.5-30 micron.
None of the aforementioned light-scattering polymer materials are entirely satisfactory in the processing of molding compounds. Often agglomeration of the scattering particles occurs during thermoplastic processing, particularly during extrusion, of molding compounds containing scattering bodies. Consequently local inhomogeneities develop in the light-scattering molded articles produced from these molding compounds.